The impressive chemistry, applications and features of ionic liquids: properties, catalysis & catalysts and trends

ACCOUNT

The impressive chemistry, applications and features of ionic liquids: properties, catalysis & catalysts and trends

Brenno A. D. NetoI, ^*; John Spencer^II

^ILaboratory of Medicinal and Technological Chemistry, Chemistry Institute, University of Brasília (UnB), Campus Universitário Darcy Ribeiro, P.O. Box 4478, 70904-970 Brasília-DF, Brazil

^IIDepartment of Chemistry, School of Life Sciences, University of Sussex, Falmer, Brighton, East Sussex, BN1 9QJ, United Kingdon

ABSTRACT

Some of our contributions to the development of catalysis in ionic liquids are described. Also, the use of ionophilic ligands and catalysts with ionic tags are presented and discussed. Due to the importance of ionic liquids, some physicochemical properties of interest and their supramolecular organization are described.

Keywords: ionic liquids, catalysis, ionic tags, mass spectrometry

RESUMO

Algumas das nossas contribuições para o desenvolvimento da área da catálise em líquidos iônicos são descritas. Além disto, o uso de ligantes ionofílicos bem como a utilização de catalisadores com "etiquetas iônicas" são apresentados e discutidos. Devido à importância dos líquidos iônicos, as suas propriedades fisicoquímicas de interesse bem como a sua organização supramolecular são temas discutidos.

1. Introduction

For many years, ionic liquids (ILs) were regarded as promising solvents and materials for a wealth of possible applications. Nowadays, however, ILs have superseded this status and have become an amazing reality. Indeed, ILs are currently used in a plethora of industrial processes, as reviewed elsewhere.¹ Nowadays, there is no doubt about the great utility of ILs in both modern chemistry and technological applications.² The importance of ILs can be easily evidenced by measuring the number of important reviews published on the subject and their related applications. Many literature surveys are available to different audiences spanning disciplines from asymmetric catalysis, homogeneous catalysis, organic synthesis, green chemistry, biotransformations, analytical chemistry, industrial applications, enzymatic reactions and others.^3-12

Despite all advances on the basic properties, comprehension and use of ILs, there are still many things left to be achieved. And, somewhat surprising, their physical and chemical properties still have many issues under debate. The definition of ILs has been a subject of much debate over the years, especially in the last decade. and there are still some contrasting definitions. In the present manuscript the term "ionic liquid" will be used to describe supramolecular structures composed entirely of ions that melt below 100 °C or typically close to room temperature. Commonly, one can find an organic cation (e.g., ammonium, phosphonium, imidazolium, pyridinium and others) with an anion of relatively low coordination strength (e.g., hexafluorophosphate, halides, acetates, bis-trifluoromethanesulfonimide, tetrafluoroborate and others).

To understand the effect of a chosen IL on a specific reaction, it is necessary to have prior knowledge of their organizational behavior, physicochemical properties and reactivity. Only then it is possible to envisage a beneficial effect over organic and catalytic reactions using these materials. The reader can find many articles covering the subjects of properties, reactivity and organization in the literature, thus our option was for a brief description on the important issues of interest to our discussion rather than focus on their properties and organizational behavior. Results from other groups will be cited only if necessary for a better understanding of a specific property, result or for illustrative comparison. Considering that the scope of the present manuscript is rather limited, the reader is urged to peruse comprehensive reviews on the subject already cited herein. Moreover, this article will place an emphasis on imidazolium chemistry, which encompasses, no doubt, a rich, attractive, important, challenging and controversial class of ILs.

Imidazolium-based ILs occupy an important part, and much studied aspect of, IL chemistry. Their structural three-dimensional organization reveals one major aspect of these salts: they display structural directionality mainly due to hydrogen bonds.^13,14 The net result is a nano-organization of inclusion compounds through ionic-pairing where the IL itself acts as an entropic driver forming a supramolecular network (aggregate formation), i.e., the so called "ionic liquid effect". It is worth highlighting that the organization is spontaneous and of considerable extension, therefore, the effect is expected to be amplified due to the supramolecular aggregate formation rather than based only on the isolated cation or anion. Scheme 1 illustrates a basic organization of imidazolium-based ILs.

The tendency towards ion-pairing, aggregation and high organization is not restricted to pure ILs. Indeed, it was possible to observe in several cases that ILs showed a beneficial and direct effect on the reaction rate (and/or selectivity) during a specific transformation. The capacity of these salts to promote reactions that are difficult to perform in classical organic solvents has already been described for many cases.^15-17 These effects can be attributed to the inherent ionic nature and self-organization promoted by these salts that are capable of (co-)promoting the formation and stabilization of ionic or polar intermediates (or transition states) through different types of ion pairing and supramolecular aggregates formation, thus resulting in a spontaneous nano-organization of the system. In other words, these salts are acting as entropic drivers to the system, which is the ionic liquid effect in action.

In this account, our investigative effort to the development of organic and catalytic reactions in imidazolium-based ILs is described, the development and application of ionically-tagged catalysts, as our contribution to a better understanding of the organization, reactivity, properties and application of this intriguing class of materials.

2. Distillation of Ionic Liquids and the Controversy of Carbenes

For long one assumed that ILs were non-volatile. Nevertheless, this property was only an assumption that has dominated the chemistry of ILs since its origins than properly asserted with scientific accuracy. Since Earle's publication¹⁸ showing that some ILs are indeed distillable, many discussions have been found about this issue. The exactly mechanism of how an IL behaves during a distillation process varies according to its specific ionic composition and possibility of carbene formation. The possibility of carbene formation cannot be discarded, but it has not been proven without doubt. In this sense, the distillation process by means of atmospheric-pressure chemical ionization mass spectrometry experiments (APCI-MS and APCI-MS/MS) was investigated in our previous work.¹⁹ Six different imidazolium-based ILs (Figure 1) were "distilled" and the species involved in the process investigated.

Results indicated that all the ILs distilled as neutral aggregates of a general formula [ImA]_n, where Im is the imidazolium cation, A is the anion and n varies from 1-3. The concentration of aggregates with n = 2 or 3 increased upon increasing the temperature. These observations allowed a mechanism proposition for the process, as shown in Scheme 2.

It is important to highlight that, indeed, there is no consensus on the mechanism involved in the distillation process. Carbene formation and a proton transfer mechanism cannot be discarded, but the evidence points towards neutral aggregates during the vaporization. The tested chiral²⁰ and functionalized IL 6 showed a similar behavior during the distillation process, thus it indicates that other functionalized (chiral or not) ILs may undergo distillation through the same mechanism. Also, we can note the presence of a methyl group in the C2-position of the imidazolium ring (ILs 1 and 2, see Figure 1) to avoid carbene formation even though, the IL is distilled.

In imidazolium-based ILs, as a consequence of the relative acidity of their C2-H hydrogens (pKa on the range of 21-23),²¹N-heterocyclic carbenes (NHC) may coexist and are, therefore, believed to act as key species. Indeed, they exert a major influence on some major IL properties and are capable of stabilizing metal complex derivatives and metal nanoparticles, and, in some cases, NHC are capable of acting as catalysts for reactions performed in "noninnocent" ILs. There have been, however, controversial evidences that NHC co-exists in pure liquids or solutions with the parent imidazolium ions. Actually, many authors remain skeptical about the role of NHC in IL chemistry, particularly in protic solvents.²²

With this in mind, we have synthesized a di-charged imidazolium-based nickel-containing IL with high crystalline organization (Scheme 3) to a mass spectrometry (MS) study,²³ mostly based on electrospray ionization tandem mass spectrometry ESI-MS(/MS).

The di-charged Ni-containing IL displayed a well-organized 3D network and formed well-ordered ionic channels (Figure 2). It is noted that both imidazolium rings are directed to the nickel-containing anion.

NHC are neutral species. A major limitation of MS is that this technique is blind to neutral entities. Nevertheless, the second imidazolium in the structure of IL 8 can act as a charge-tag, thus allowing the detection and characterization of the carbene, as shown in Scheme 4.

Note that without the charge-tag, it would not be possible the detection and characterization of the NHC with a m/z 235. This successful idea was later fully explored by Corilo et al.²⁴ to probe the NHC formation in doubly, triply and quadruply charged imidazolium derivatives. Despite the good results obtained with the strategy of a positive charge-tag, the possibility that NHCs were formed in the gas phase from a direct reaction of the imidazolium and the anion could not be completely dismissed.

Since the strategy of using charge-tags is very promising and the study in the positive ion mode (ESI(+)-MS/MS) does not discard the possibility of NHC formation in a gas phase reaction, we decided to study the NHC formation in the negative ion mode by using a negative charge-tag instead of a positive one.²⁵ Thus, two imidazolium derivatives were synthesized with the possibility of forming a negative charge in the side chain of their structures (Scheme 5).

The presence of a negative charge-tag in both NHC 11 (m/z 139) and NHC 12 (m/z 203), allowed the detection and characterization by ESI(-)-MS/MS. Moreover, the study was conducted in a protic solvent (i.e., methanol) to show that even in protic solvent NHCs can co-exist under many conditions. ESI was, in this case, an excellent choice since it is a technique capable of a gentle "fishing" of the ions from the solution directly into the gas phase.²⁶ Fortunately, the strategy proved to be valuable and both NHCs (11 and 12) were detected and characterized. To be sure about their formation, both NHCs were also promptly reacted with CO₂ in the gas phase. The ion/molecule reactions, which is a very elegant way to confirm the proposed structures, resulted in the direct addition of NHC to CO₂ (Scheme 6), thus confirming the interception of the NHC species.

Indeed, all results opened up a new avenue for the intrinsic physicochemical properties evaluation and the solvent/counter-ion reactivity of many types of reactants with gaseous NHC from imidazolium derivatives, which are now possible, based on the strategy of negative charge-tags.

3. Supramolecular Aggregates (and Ion-Pairing) Formation: the Ionic Liquid Effect

The capacity of forming supramolecular aggregates with both cations and anions allows the possibility of stabilizing charged and polar intermediates (or transition states) of many kinds of reactions by using ILs as the media.

In this sense, the ionic nature of the intermediary species in the Baylis-Hillman reaction (BHR) render them as an attractive model for studying the effect of ionic media on both the stability and reactivity of such intermediates, as shown in the commonly accepted mechanism for the BHR (Scheme 7).

Indeed, the use of ESI-MS allowed us to detect and characterize the supramolecular aggregates from the BHR (Figure 3).²⁷

These unprecedented supramolecular species (Figure 3) detected by both modes ESI(+)MS(/MS) and ESI(-)MS(/MS) allowed a better understanding on how imidazolium-based ILs could help in the formation and stabilization of the charged intermediates from the BHR. Those ILs were capable of participating as supramolecular coordination species resulting in more stable intermediates. Moreover, we proposed the aldehyde activation toward nucleophilic attack via 1-n-butyl-3-methylimidazolium (BMI) coordination (Scheme 8).

We have also studied the ionic liquid effect in intermolecular hydroamination or hydroarylation reactions of norbornene and cyclohexadiene performed with catalytic amounts of BrØnsted or Lewis acid in imidazolium ILs (Scheme 9).²⁸ It is worth remembering that BrØnsted acids commonly display superacid behavior in ILs, as reviewed elsewhere.²⁹

Results showed higher selectivity and yields for those reactions performed in ILs than those performed in classical organic solvents. The IL increases the acidity of the media and stabilizes ionic intermediates through the formation of supramolecular aggregates. The presence of charged intermediates is clear from Scheme 9 in the proposed mechanism. It is also possible to observe that the anion may play a role in the stabilization of these intermediates. In fact, it was possible to conclude that anions with low coordination ability (related to the basic character of the anion and the ionicity of the molten salt), i.e., such as NTf₂, gave better results for the reaction. ILs not only increased the acidity of the media, especially of the anilinium salt, but also stabilized the key intermediate ionic species via the formation of supramolecular aggregates. In ionic media, it was also possible to tune the selectivity for N-H or Ar-H products by changing the reaction conditions and aniline substitution patterns. Supramolecular aggregates were detected and characterized by ESI(+)-MS(/MS), demonstrating the ability of the anion to stabilize the charged intermediates formed during the transformation. Indeed, the ionic liquid effect resulted in much better yields and selectivity when compared to those in classical molecular solvents. Figure 4 shows the detected supramolecular aggregates.

4. Catalysis in Imidazolium-Based Ionic Liquids

ILs are very attractive media to promote catalytic reactions. Their negligible vapor pressure, capacity to solubilize a wild range of organic, inorganic and organometallic compounds, tuned miscibility (or immiscibility) with a plethora of solvents, and tuned physicochemical properties make ILs an extremely attractive media to promote a wild range of catalytic reactions. Moreover, in many cases, it is interesting to note the formation of two-phase catalytic systems, which are commonly observed in catalysis promoted in ILs. Many recently reviews describe some aspects of the advances made in IL-mediated catalysis.^30-32

Considering all these attractive features of imidazolium ILs, they were thought as alternatives to organoaluminate melts, which were prepared upon mixing of quaternary ammonium salts (e.g. N-alkylpyridinium or 1,3-dialkylimidazolium halides) with AlCl₃ in different proportions. However, these compounds are reactive towards air and moisture. Moreover, they are difficult to handle and several organic/organometallic substrates are not chemically inert in these media. These major drawbacks limit their application, especially as recyclable media. In this sense, we have developed³³ a novel organoindate room temperature IL (Scheme 10) to overcome these limitations.

The new IL BMI.InCl₄ displayed relative low viscosity, was easy to handle, air stable and not moisture sensitive. The new ionic liquid was applied in the tetrahydropyranylation of different alcohols (Scheme 11) and it was used as the reaction media and catalyst for the reaction.

The system was recycled and reused five times. One impressive feature of this reaction is the product separation (Scheme 12).

Scheme 12

The Mannich reaction and the N-acyliminium chemistry are very useful and can be applied in the synthesis of many alkaloids and biologically active compounds. N-acyliminium cations can be generated in the presence of different Lewis acids. Since the IL BMI.InCl₄ had already a Lewis acid character in its anion, we successfully combined such a partnership to test an in situ generation of those ions which were trapped by active olefins (Scheme 13).³⁴

Interestingly, the use of other ILs rather than BMI.InCl₄ gave no interesting result. The test reaction using BMI.BF₄ and 5 mol% of InCl₃ (as an additive) gave the desired product in only 25% at 50 °C. On the other hand, reactions carried out in BMI.InCl₄ without any additive, resulted in compound 17 in 80% at room temperature (25 °C). The diastereoselectivity of the reaction was evaluated using Z-olefins and gave very good results. All diastereoselectivities ranged from 5:1 to 12:1 favoring the erythro isomers.

Another application of BMI.InCl₄ was to support different metal catalysts to promote biodiesel synthesis. We have used this IL as support with impressive results.³⁵ The best catalyst among the 28 tested was Sn(3-hydroxy-2-methyl-4-pyrone)₂(H₂ O)₂ supported in BMI.InCl₄ to perform the transesterification reaction of soybean oil (Scheme 14).

We found that the biodiesel yield increased from 55 to 83% when the IL was changed from BMI.PF₆ to BMI.InCl₄ within just 4 h. After this time, yields decrease as a consequence of a reverse transesterification with the formed glycerol. We have also showed that the catalytic system was active for a direct esterification reaction. Based on all obtained results, the mechanism could be studied and a catalytic cycle for the transformation was proposed (Scheme 15).

Upon methanol addition, the catalytically active species 20 is formed in situ through a ligand exchange. Thus, the coordination of the triglyceride (or diglyceride, monoglyceride or the fatty acid) takes place forming the intermediate 21. As a consequence, the natural polarization of the C=O bond increase, facilitating the next step, i.e., the nucleophilic attack of the alcohol. After that, species 21 probably develops via a four center transition state 22 furnishing the desired biodiesel, as shown in Scheme 15.

Commonly used ILs were also tested as support to classic acid and base catalyzed synthesis of biodiesel. We showed the efficiency of the catalytic system with K₂CO₃ (basic catalysis) and H₂SO₄ (acid catalysis) in those ILs.³⁶ The biodiesel could be obtained in almost quantitative yields in less than 1 h. One interesting feature of the article was the study on carbene formation in BMI.NTf₂, since there was the presence of basic species to promote the biodiesel synthesis. By using ¹³C NMR, we could observe that the presence of ethanol inhibited the carbene formation (Figure 5).

It is noted that the addition of a base resulted in a drastic reduction of the C2 carbon intensity in the ¹³C spectrum indicating carbene formation. In the presence of ethanol, the signal increases again, indicating that the carbene may form, but the presence of an acidic hydrogen in the alcohol allows an immediate protonation.

Biodiesel, despite being a promising alternative fuel, has many technological problems associated with its use. ILs, however, have proven to be an excellent media for the synthesis and modification of this biofuel, as recently reviewed.^37-40 In this sense, we decided to perform an enzymatic modification of biodiesel towards a better oxidative stability of the biofuel.⁴¹ We have tested 9 commercially available lipases to promote the epoxidation of C=C bond of methyl oleate using hydrogen peroxide as the oxidizing agent (Scheme 16).

Interestingly, the three tested ILs (BMI.PF₆, BMI.NTf₂ and BMI.BF₄) gave very different results. The use of hydrophobic ILs (BMI.PF₆ and BMI.NTf₂) gave the desired epoxidized methyl oleate (EMO) in good yields. However, in BMI.PF₆, the best enzyme was R Amano K (from Penicillium roqueforti) which resulted in EMO in 82% in the fifth reaction hour. In the mean time, the best enzyme in BMI.NTf₂ was lipase type II (from calf tongue roof), resulting in EMO in 78% in the first hour of reaction. It was also observed low yields of DIOL with those two ILs. The use of a hydrophilic IL, however, gave the most surprising and impressive result. In the first hour of reaction, only EMO was obtained in 89% by using lipase Amano A (from Aspergillus niger). After this time, the DIOL yield increased as well, as observed in Figure 6. We monitored the reaction for 5 h using lipase Amano A.

The best DIOL yields were observed in BMI.BF₄ too. The hydrophilic character of this IL allowed a better dissolution of the oxidant agent (H₂O₂) in the reaction media. In the third reaction hour, DIOL was obtained in 60% (the best DIOL yield among the tested ILs).

ILs can be used as efficient media to support and stabilize metal nanoparticles and this subject has been reviewed.^42-45 In this sense, we have tested in situ generated palladium nanoparticles stabilized in BMI.PF₆ and BMI.BF₄ to promote the selective and partial hydrogenation of biodiesel⁴⁶ envisaging an increase in its oxidative stability.

The use of imidazolium-based ILs allowed the formation of ca. 5 nm palladium nanoparticles. Indeed, the catalytic system proved to be the most selective and efficient system for performing the selective hydrogenation of biodiesel towards oleic composition of the final product. This effect was attributed to the solubility of mono-enes and dienes in ILs. It is known that commonly dienes have a high solubility in ILs when compared to mono-enes. Thus, a mechanism to the partial hydrogenation could be proposed, as shown in Scheme 17.

Another important feature was the transmission electron microscopy (TEM) analyses after catalysis, which revealed no particle aggregation. However, the catalytic system lost its activity upon recycling. BET analysis was then performed with very elucidative results. After the first run a specific surface area of 171.685 m² g^-1, a porous volume of 0.3854 cc g^-1 and a porous size smaller than 735.8 Å were noted. After the sixth run, however, a surface area of 199.615 m² g^-1, a porous volume of 0.4726 cc g^-1 and a porous size smaller than 412.8 Å were observed. Considering that the pore size decreasing during each runs, it was comprehensive the reason for the loss of activity of the nanostructure system. The substrate could not access the catalyst preventing the reaction from taking place, especially because biodiesel is much higher in its van der Waals volume.

Biologically active compounds, such as isatins,⁴⁷ were synthesized in imidazolium-based ILs with impressive results. Upon anion exchange, it was possible for us to obtain the isatin-3-oxime derivatives⁴⁸ under acid catalysis with considerable different yields (Scheme 18).

The effect was attributed to the stabilization observed in the charged intermediates of the cyclization through ion-pairing formation, as shown in Scheme 19. The best results were obtained using BMI.NTf₂ as the ionic media and HBF₄ as the BrØnsted acid.

Besides the anion effect, it is worth remembering that BrØnsted acids have their acid strength increased in imidazolium-based ILs,^29,49 as discussed before.

Amides belong to a very important class of compounds. The amide group is widespread and found in a plethora of natural products, polymers, signaling molecules and others. The class of large-chain amides is of special interest due to their biological activity. The synthesis of amides, however, is a not an easy task, as recently reviewed.⁵⁰ Envisaging an efficient methodology to obtain large-chain biomass amide derivatives, we supported some Lewis and BrØnsted acids in ILs to obtain the desired product from an aminolysis reaction esters and carboxylic acids (Scheme 20).⁵¹ Reactions conducted in organic solvents (or solventless) gave only reasonable yields with longer reaction times.

All tested ILs (BMI.NTf₂, BMI.PF₆ and BMI.BF₄) gave essentially quantitative yields using BF₃.OEt₂ as the catalyst (5 mol%) and methyl oleate as the model substrate in the biphasic catalytic system. However, to avoid the anion degradation (PF₆^- and BF₄^-), we decided to keep the study using only BMI.NTf₂ as the ionic media. HCl, H₂SO₄, SnCl₂ and CdO gave by far the best results (ca. quantitative yields). Since we could not recycle BF₃.OEt₂, HCl and H₂SO₄, we decided to study the transformation with those two metals, i.e., SnCl₂ and CdO. Both catalysts could be used at least 8 times without any loss in the catalytic activity. CdO, nevertheless, underwent leaching and 1% of the metal could be found in the product phase (upper phase). The methodology allowed the synthesis of many amide derivatives in good to excellent yields.

The mechanism of the transformation and the ionic liquid effect were studied by NMR and ESI-MS(/MS) experiments. All results allowed a better understanding on the need of amine excess in the reaction media (Scheme 21).

The obtained results indicated the preference for a 6-membered transition state, thus explaining the excess of amine in the reaction media. Moreover, the C=O bond activation was through a cooperative mechanism of the IL and the metal catalyst, thus explaining the importance of the IL in the reaction. In this case, the used IL was not only responsible for stabilize the polar and charged intermediates, but it was also responsible for a partial activation of the carbonyl group in the substrate. Due to all features studied in this transformation, it was possible to conclude that ionic liquid effect was more than just the stabilization of the charged and polar intermediates, rather the imidazolium cation also allowed a better activation of the ester (or the acid) to initiate the reaction in the presence of a metal catalyst.

5. Catalysts with Ionic Tags

Many advantages in the use of catalysts with ionic tags have been shown. Functionalized liquids or task-specific ionic liquids (TSILs) play a fundamental role in modern catalysis, as reviewed.^31,52-54

Envisaging a more efficient support (anchoring) in the ionic phase, ligands (and their metal derivatives) with ionic tags have been developed and tested, especially in biphasic catalysis. Aiming to achieve higher selectivities and activities (advantages of homogeneous catalysis) associate with easier product separation, catalyst recovery and reuse (advantages of heterogeneous catalysis) ILs are an obvious choice for the use of ionically-tagged catalysts. Since ILs are inherently "ionic" entities, it is more than reasonable that the tethering of an ionic tag on the catalyst structure increases the affinity (and solubility) for the IL phase. Therefore, one should expect an efficient retention of the catalyst in the ionic phase even after an extraction, thus rendering the process more environmentally benign, green, economically viable and sustainable. Moreover, an additional feature of the presence of an ionic tag is that water solubility is commonly increased,⁵⁵ and it may be a desirable characteristic in many cases.

Indeed, it is interesting to rationalize some important features, principles and advantages on the use of ionically-tagged catalysts:

(i) Increased solubility in an IL phase as a consequence of the chemical affinity between the tagged ligand (or tagged metal catalyst derivative) and the ionic media.

(ii) Efficient support due to the strong interaction with the IL phase, thus avoiding the catalyst leaching and possible recovers of it.

(ii) Improvement on the catalyst physicochemical properties such as thermal stability and electrochemical window. The use of an ionic tag is an elegant and straightforward manner to tune the properties of a specific catalyst by turning it more similar to an IL.

(iii) The possibility of a nano-organization as a consequence of the ionic tag. It is worth remembering that imidazolium derivatives may act as "entropic drivers"⁴⁴ with nano-effects and organization.

(iv) A chemical role by using catalysts with ionic tags as proposed by Lombardo and Trobini:⁵³ "We posit as a working hypothesis that, if the tag ion pair can approach charges that develop along the reaction coordinate with minimal distortion of bond angles and distances, it can lower the free-energy barrier by complementing charge separation in the dipolar transition state. As a consequence, the catalyst loading can be reduced compared to the reference homogeneous catalyst."

(v) Orientation for the chemical approach of different reagents/reactants due the possibility of aggregate or ion-pairing formation with polar and charged intermediates. Moreover, as a consequence, the transition state of this kind of reaction is directly affected by the charge (ionic tag) in the catalyst structure. Therefore, different selectivities and yields are observed (normally both are increased).

(vi) As already described by Sebesta et al.⁵⁴ "an important premise being that the ionic tag is catalytically silent and inert in reaction conditions." However, despite it is a general principle, not always it can be assumed as a true and, in some cases, the noninnocent⁵⁶ nature of an imidazolium tag may be a desirable feature, as will be shown.

With all these features in mind, we have designed the synthesis and application of new catalysts with at least one ionic tag in their structure. Initially, we decided to keep the strategy of a charge tag to investigate metal complexes of palladium, copper and nickel and apply the ionically-tagged palladium complex as the promoter of phosphine-free Heck and Suzuki reactions.⁵⁷

First, the known⁵⁸ ionically-tagged ligand (and acetate derivative) was synthesized as shown in Scheme 22.

The ligand MAI.Cl could then be treated in situ with neutral Cu(OAc)₂, Ni(OAc)₂ and Pd(OAc)₂ to form mono- (and di-)-charged complex (Scheme 23).

Studies in the gas-phase by ESI-MS(/MS) revealed a unique chemistry of this species, especially for Cu and Ni derivatives, as shown in Scheme 24.

Of great importance was the detection and characterization of the organometallic derivatives 27a,b. The cyclization reaction (Scheme 24, pathway A) allowed the formation of a more stable organometallic derivative through the reaction with the in situ recently formed carbene from the imidazolium moiety. The reaction with Pd(OAc)₂, however, showed unique features in the gas phase, as shown in Scheme 25.

The presence of an acetate anion directly coordinated in the palladium center is in full accordance with the proposition of a negative palladium as an intermediate in Pd cross-coupling reactions.⁵⁹ In solution, however, NMR experiments indicated the in situ formation of the organometallic palladium derivative (Scheme 26). The ionically-tagged palladium complex was synthesized (Scheme 26), characterized and successfully applied as the promoter of a phosphine-free version of the Suzuki and Heck reactions (Scheme 26) with good results.

Unfortunately, reactions conducted in ILs gave poor results for both the Heck and Suzuki reactions, probably as a consequence of the catalyst inactivation due to carbene formation in BMI.BF₄ (or BMI.PF₆ or BMI.NTf₂) in the presence of different bases. Even though, the combination of a classic molecular solvent such as methanol and weak bases (e.g., K₃PO₄ or K₂CO₃) with the catalyst 29 proved to be a very active system for those phosphine-free reactions. For instance, in the first reaction hour, the Heck adduct was obtained in 85% and, under the same reaction conditions, the use of Pd(OAc)₂ gave the same adduct in only 6% yield after the same time.

Very recently, we have synthesized and applied a novel ionically-tagged iron complex (Scheme 27) to promote the epoxidation of C=C bonds for biomass derivatives.⁶⁰

The novel catalyst 34 was tested in the epoxidation reaction of methyl oleate to afford EMO and DIOL (as seen in Scheme 16). The use of BMI.PF₆ and BMI.BF₄, as the reaction media, despite the good results (high yields), was not a good choice due to anion degradation (HF release) under the catalysis conditions, thus the best ionic media was BMI.NTf₂. Air (oxygen) and hydrogen peroxide (30% v/v) were tested as the oxidizing agents with success. The use of H₂O₂, nevertheless, avoided any recycle reaction due to catalyst inactivation despite the mild conditions (30 °C). On the other hand, the use of air as the oxidant at 90 °C, allowed the catalyst and media recovering and reutilization at least ten times without any loss in the catalyst activity. ICP-AES revealed that down to 2 ppm of iron could be found in the oil phase. The catalytic system was successfully applied with other biomass substrates and, in all cases, the epoxidized compound was obtained in high yields (Figure 7).

Another important feature of this work was the mechanistic study, which was mostly based on ESI-MS(/MS) experiments. It was noticed that reactions performed with H₂O₂ preferentially developed through a radical mechanism and, in the meantime, the air oxidation preferred a concerted mechanism (Scheme 28).

The detected and characterized intermediates have a direct influence on the comprehension of cytochrome chemistry and indicated some possibilities of action of those enzymes during the enzymatic-catalyzed oxidation processes.

6. Summary and Outlook

Despite all advances obtained with ILs, there are still many challenges to be overcome, drawbacks to be transposed and questions to be answered. The importance and potential of ILs achieved a level that no one could ever expect in the beginning of their development. Some important physicochemical parameters such as polarity are still under debate and others, like their distillation process, is so-far indisputable. The importance of carbenes as active catalytic species have been already reported,^61-63 especially those derived of 1,3-dialkylimidazolium cation.^63-67 Even though, there are many challenges surrounding the carbene chemistry and its potential applications in the field of organo- and organometallic catalysis. Without doubt, ILs are nowadays in front of a large and promising avenue towards catalysis development. Thus, the search for greener, more selective, faster and cheaper processes, necessarily points to the development of ILs their applications.

In summary, IL chemistry continues to advance with more applications in fundamental and applied processes leading to technological progress and economic impact. This has been a direct consequence of our improved understanding of the role of ILs, by the use of tags, mass spectrometry, etc. and the application of ILs to real problems in the real world where often current approaches fall short.

Acknowledgements

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Financiadora de Estudos e Projetos (FINEP), Fundação de Empreendimentos Científicos e Tecnológicos (FINATEC), Fundação de Apoio à Pesquisa do Distrito Federal (FAPDF) and Decanato de Pesquisa e Pós-Graduação (DPP-UnB) are acknowledged for partial financial support. B. A. D. Neto also thanks CNPq for the research fellowship.

Submitted: April 24, 2012

Published online: May 22, 2012

Prof. Brenno A. D. Neto received his degree in Chemistry from the Federal University of Rio Grande do Sul (UFRGS, Rio Grande do Sul State, Brazil). In 2003, he completed his MSc at the same University, and in 2006, completed his PhD under the supervision of Prof. JaÏrton Dupont and Prof. Valentim E. U. Costa. After a period as a post-doc at the Parque Científico e Tecnológico (TECNOPUC, Rio Grande do Sul State) and as a Reader at the Pontifical Catholic University of Rio Grande do Sul (PUCRS), he became a Professor at the Chemistry Institute of the University of Brasília (UnB). His research interests are centered on photoluminescent selective cell markers and optoelectronics, synthesis and modification of biofuels, catalysis and reactions in ionic liquids.

Prof. John Spencer graduated in Chemistry with French from University of Sussex (United Kingdom) in 1990. He carried out his PhD at the Université de Strasbourg (ULP, Strasbourg, France, under the supervision of Prof. Michel Pfeffer) and, after a postdoctoral stay with Prof. Antonio Togni (Swiss Federal Institute of Technology Zurich, ETH, Zurich, Switzerland), he spent ten years in the pharmaceutical sector culminating in five years at the James Black Foundation (United Kingdom). He has a long history in palladacycle chemistry, Suzuki couplings and the combination of these with microwave and parallel synthesis to make libraries of novel compounds.

• 1. Plechkova, N. V.; Seddon, K. R.; Chem. Soc. Rev. 2008, 37, 123.
• 2. Zhao, H.; Malhotra, S. V.; Aldrichim. Acta 2002, 35, 75.
• 3. Tariq, M.; Freire, M. G.; Saramago, B.; Coutinho, J. A. P.; Lopes, J. N. C.; Rebelo, L. P. N.; Chem. Soc. Rev. 2012, 41, 829.
• 4. Sheldon, R. A.; Chem. Soc. Rev. 2012, 41, 1437.
• 5. Hallett, J. P.; Welton, T.; Chem. Rev. 2011, 111, 3508.
• 6. Scholten, J. D.; Leal, B. C.; Dupont, J.; ACS Catal. 2012, 2, 184.
• 7. Dupont, J.; de Souza, R. F.; Suarez, P. A. Z.; Chem. Rev. 2002, 102, 3667.
• 8. Petkovic, M.; Seddon, K. R.; Rebelo, L. P. N.; Pereira, C. S.; Chem. Soc. Rev. 2011, 40, 1383.
• 9. Welton, T.; Chem. Rev. 1999, 99, 2071.
• 10. van Rantwijk, F.; Sheldon, R. A.; Chem. Rev. 2007, 107, 2757.
• 11. Martins, M. A. P.; Frizzo, C. P.; Moreira, D. N.; Zanatta, N.; Bonacorso, H. G.; Chem. Rev. 2008, 108, 2015.
• 12. Suarez, P. A. Z.; Consorti, C. S.; de Souza, R. F.; Dupont, J.; Goncalves, R. S.; J. Braz. Chem. Soc. 2002, 13, 106.
• 13. Dupont, J.; Acc. Chem. Res. 2011, 44, 1223.
• 14. Dupont, J.; J. Braz. Chem. Soc. 2004, 15, 341.
• 15. De Bellefon, C.; Pollet, E.; Grenouillet, P.; J. Mol. Catal. A: Chem. 1999, 145, 121.
• 16. Dubreuil, J. F.; Bazureau, J. P.; Tetrahedron Lett. 2000, 41, 7351.
• 17. Chauvin, Y.; Mussmann, L.; Olivier, H.; Angew. Chem., Int. Ed. 1996, 34, 2698.
• 18. Earle, M. J.; Esperanca, J.; Gilea, M. A.; Lopes, J. N. C.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A.; Nature 2006, 439, 831.
• 19. Neto, B. A. D.; Santos, L. S.; Nachtigall, F. M.; Eberlin, M. N.; Dupont, J.; Angew. Chem., Int. Ed. 2006, 45, 7251.
• 20. Prechtl, M. H. G.; Scholten, J. D.; Neto, B. A. D.; Dupont, J.; Curr. Org. Chem. 2009, 13, 1259.
• 21. Amyes, T. L.; Diver, S. T.; Richard, J. P.; Rivas, F. M.; Toth, K.; J. Am. Chem. Soc. 2004, 126, 4366.
• 22. Mi, X. L.; Luo, S. Z.; Cheng, J. P.; J. Org. Chem. 2005, 70, 2338.
• 23. Kuhn, M. C. A.; Lapis, A. A. M.; Machado, G.; Roisnel, T.; Carpentier, J. F.; Neto, B. A. D.; Casagrande, O. L.; Inorg. Chim. Acta 2011, 370, 505.
• 24. Corilo, Y. E.; Nachtigall, F. M.; Abdelnur, P. V.; Ebeling, G.; Dupont, J.; Eberlin, M. N.; RSC Adv. 2011, 1, 73.
• 25. Lalli, P. M.; Rodrigues, T. S.; Arouca, A. M.; Eberlin, M. N.; Neto, B. A. D.; RSC Adv. 2012, 2, 3201.
• 26. Coelho, F.; Eberlin, M. N.; Angew. Chem., Int. Ed. 2011, 50, 5261.
• 27. Santos, L. S.; Neto, B. A. D.; Consorti, C. S.; Pavam, C. H.; Almeida, W. P.; Coelho, F.; Dupont, J.; Eberlin, M. N.; J. Phys. Org. Chem. 2006, 19, 731.
• 28. Lapis, A. A. M.; Neto, B. A. D.; Scholten, J. D.; Nachtigall, F. M.; Eberlin, M. N.; Dupont, J.; Tetrahedron Lett. 2006, 47, 6775.
• 29. Johnson, K. E.; Pagni, R. M.; Bartmess, J.; Monatsh. Chem. 2007, 138, 1077.
• 30. Wong, W. L.; Wong, K. Y.; Can. J. Chem. 2012, 90, 1.
• 31. Sawant, A. D.; Raut, D. G.; Darvatkar, N. B.; Salunkhe, M. M.; Green Chem. Lett. Rev. 2011, 4, 41.
• 32. Zhang, Q. H.; Zhang, S. G.; Deng, Y. Q.; Green Chem. 2011, 13, 2619.
• 33. Neto, B. A. D.; Ebeling, G.; Goncalves, R. S.; Gozzo, F. C.; Eberlin, M. N.; Dupont, J.; Synthesis 2004, 1155.
• 34. Pilli, R. A.; Robello, L. G.; Camilo, N. S.; Dupont, J.; Lapis, A. A. M.; Neto, B. A. D.; Tetrahedron Lett. 2006, 47, 1669.
• 35. Neto, B. A. D.; Alves, M. B.; Lapis, A. A. M.; Nachtigall, F. M.; Eberlin, M. N.; Dupont, J.; Suarez, P. A. Z.; J. Catal. 2007, 249, 154.
• 36. Lapis, A. A. M.; de Oliveira, L. F.; Neto, B. A. D.; Dupont, J.; ChemSusChem 2008, 1, 759.
• 37. Liu, C. Z.; Wang, F.; Stiles, A. R.; Guo, C.; Appl. Energy 2012, 92, 406.
• 38. Earle, M. J.; Plechkova, N. V.; Seddon, K. R.; Pure Appl. Chem. 2009, 81, 2045.
• 39. Cordeiro, C. S.; da Silva, F. R.; Wypych, F.; Ramos, L. P.; Quim. Nova 2011, 34, 477.
• 40. Narayan, M. P.; Kumar, S. S.; Arunrao, A. S.; Karthikeyan, P.; Rambhau, B. P.; Res. J. Chem. Environ. 2011, 15, 92.
• 41. Silva, W. S. D.; Lapis, A. A. M.; Suarez, P. A. Z.; Neto, B. A. D.; J. Mol. Catal. B: Enzym. 2011, 68, 98.
• 42. Hagiwara, H.; Heterocycles 2012, 85, 281.
• 43. Chaturvedi, D.; Curr. Org. Chem. 2011, 15, 1236.
• 44. Dupont, J.; Scholten, J. D.; Chem. Soc. Rev. 2010, 39, 1780.
• 45. Zahmakiran, M.; Ozkar, S.; Nanoscale 2011, 3, 3462.
• 46. Carvalho, M. S.; Lacerda, R. A.; Leao, J. P. B.; Scholten, J. D.; Neto, B. A. D.; Suarez, P. A. Z.; Catal. Sci. Technol. 2011, 1, 480.
• 47. da Silva, J. F. M.; Garden, S. J.; Pinto, A. C.; J. Braz. Chem. Soc. 2001, 12, 273.
• 48. Pinto, A. C.; Lapis, A. A. M.; da Silva, B. V.; Bastos, R. S.; Dupont, J.; Neto, B. A. D.; Tetrahedron Lett. 2008, 49, 5639.
• 49. Chiappe, C.; Rajamani, S.; Eur. J. Org. Chem. 2011, 5517.
• 50. Allen, C. L.; Williams, J. M. J.; Chem. Soc. Rev. 2011, 40, 3405.
• 51. de Oliveira, V. M.; de Jesus, R. S.; Gomes, A. F.; Gozzo, F. C.; Umpierre, A. P.; Suarez, P. A. Z.; Rubim, J. C.; Neto, B. A. D.; ChemCatChem 2011, 3, 1911.
• 52. Lee, S. G.; Chem. Commun. 2006, 1049.
• 53. Lombardo, M.; Trombini, C.; ChemCatChem 2010, 2, 135.
• 54. Sebesta, R.; Kmentova, I.; Toma, S.; Green Chem. 2008, 10, 484.
• 55. Shaughnessy, K. H.; Chem. Rev. 2009, 109, 643.
• 56. Dupont, J.; Spencer, J.; Angew. Chem., Int. Ed. 2004, 43, 5296.
• 57. Oliveira, F. F. D.; dos Santos, M. R.; Lalli, P. M.; Schmidt, E. M.; Bakuzis, P.; Lapis, A. A. M.; Monteiro, A. L.; Eberlin, M. N.; Neto, B. A. D.; J. Org. Chem. 2011, 76, 10140.
• 58. Li, J. Z.; Peng, Y. Q.; Song, G. H.; Catal. Lett. 2005, 102, 159.
• 59. Amatore, C.; Jutand, A.; Acc. Chem. Res. 2000, 33, 314.
• 60. dos Santos, M. R.; Diniz, J. R.; Arouca, A. M.; Gomes, A. F.; Gozzo, F. C.; Tamborim, S. M.; Parize, A. L.; Suarez, P. A. Z.; Neto, B. A. D.; ChemSusChem 2012, 5, 716.
• 61. Bugaut, X.; Glorius, F.; Chem. Soc. Rev. 2012, 41, 3511.
• 62. Fortman, G. C.; Nolan, S. P.; Chem. Soc. Rev. 2011, 40, 5151.
• 63. Phan, N. T. S.; Van Der Sluys, M.; Jones, C. W.; Adv. Synth. Catal. 2006, 348, 609.
• 64. Monge-Marcet, A.; Pleixats, R.; Cattoen, X.; Man, M. W. C.; Catal. Sci. Technol. 2011, 1, 1544.
• 65. Liu, S. F.; Xiao, J. L.; J. Mol. Catal. A: Chem. 2007, 270, 1.
• 66. Giernoth, R. In In Situ Nmr Methods in Catalysis, vol. 276; Bargon, J.; Kuhn, L. T., eds.; Springer-Verlag: Heidelberg, Germany, 2007, p. 1.
• 67. Lin, I. J. B.; Vasam, C. S.; J. Organomet. Chem. 2005, 690, 3498.

Publication Dates

• Publication in this collection
  13 July 2012
• Date of issue
  June 2012

History

• Received
  22 May 2012
• Accepted
  22 May 2012